All mammals feed their young with milk, but that milk is rich in lactose rather than sugars we see in other foods (namely sucrose, or table sugar). This isn’t likely to be an accident: lactose is made up of galactose and glucose, and not only is galactose a major nutrient for newborns,[1] it is important for their development.[2]
Galactose is also part of complex molecules that are essential to the nervous system and immunity. Given that infants have increased nutritional needs, in order to bolster their developing physiology, their intake of lactose makes sense.